One of the most thorough investigations into referee bias has found that they tend to award harsher foul punishments to the away team. The new results, published in the International Journal of Sport and Exercise Psychology, suggest that experienced referees are just as prone to this bias as their less experienced colleagues.

Andrés Picazo-Tadeo and his team analysed data from 2,651 matches played in the First Division of La Liga, the Spanish Football League between the 2002/3 and 2009/10 seasons, inclusive. Unlike previous research, they were careful to consider the referees’ foul decisions separately from the awarding of penalty cards (given as punishment for serious fouls). It’s been shown before that referees tend to award more free kicks and cards in favour of the home team, but this is not strong evidence for a home team bias because it’s possible that away teams simply tend to commit more fouls. The new research specifically looks not just at the distribution of referees’ foul decisions between home and away teams, but it also examines separately how harshly referees punish any fouls.

In fact, the research uncovered no difference in the number of fouls that referees attributed to home and away teams. But after a foul, referees tended to punish away teams more harshly with more yellow and red cards, and this was especially the case when the home crowd was larger. The presence of a running track between the pitch and the crowd made no difference, and as mentioned, neither did referee experience. The basic result complements a recent lab study that also found that simulated crowd noise influenced referees to punish fouls more severely.

Picazo-Tadeo and his colleagues speculate that perhaps referees’ initial foul decisions are made relatively automatically, in the heat of unfolding play, thus making them immune to social pressure from the home crowd. In contrast, after play has halted, the referee has time to decide on the severity of the infringement and here the noise of the crowd may sway their thinking – indeed, they may even, without realising they are doing it, use the noise of the crowd as a cue for the seriousness of the foul. This would inevitably bias their decisions against the away team because of the noisy protests of the larger home crowd whenever one of their players was the victim of a foul.

An important caveat is that although the study took account of the number of fouls made by each team, the researchers don’t have any objective measure (beyond the referees’ card decisions) of the actual seriousness of the fouls committed. It’s possible that away teams tend to commit more serious fouls than home teams, which if true would undermine the results.

Notwithstanding this possibility, the researchers said their results suggest that local supporters can influence referee decisions after a foul has been called. “One recommendation for supporters is that they should exert more social pressure in the moments immediately after a referee indicates that the away team has committed a foul,” they said. Meanwhile, they recommended that referee training incorporate lessons on how to ignore irrelevant cues, such as crowd noise.